1. Field of the Invention
The present invention relates to a method-for manufacturing a wiring circuit board which has bumps of a constant fixed height.
2. Description of the Related Art
Very small bumps (e.g., 50 μm in diameter and 30 μm in height) are widely used in connections between electronic component elements such as semiconductor or LCD elements and wiring circuit boards, or between the different layers in multilayered wiring substrates.
A representative method for manufacturing bumps of such size is given in FIGS. 4A to 4E.
In other words, first, a dual-layer flexible substrate 43 comprising a copper foil 42 affixed to a polyimide film 41 is prepared, as shown in FIG. 4A, after which the copper foil 42 is patterned using a photolithography technique to form a wiring circuit 44 (FIG. 4B).
Next, a cover coat layer 45 is formed on the wiring circuit 44 in accordance with a conventional method (FIG. 4C). For example, it is possible to form the cover coat layer 45 by forming a polyamic acid layer on the wiring circuit 44, patterning it using photolithography and imidising it. A resist ink can also be printed thereon.
Next, very small bumps can be formed by irradiating with a laser light the region of the polyimide film 41 which corresponds to the wiring circuit 44 to form bump holes 46 (FIG. 4D), and then, using an electrolytic plating method to form metal bumps 47 on the wiring circuit 44, which is exposed in the bottom portion of the bump holes 46. The cover coat layer 45 may be optionally covered with a protective film (not shown) after the irradiation and before the electrolytic plating (FIG. 4E).
However, when the bump holes 46 are formed by the irradiation with the laser light, the surface area of the openings will vary, owing to scattering in the amount of smearing attached to the bottom portion of the bump holes 46; therefore a drawback arises in that substantial scattering appears in the height of the metal bumps 47. Accordingly, stable bump connections become difficult to achieve. Connecting semiconductor elements to the wiring circuit in one step using ultrasonic waves becomes particularly difficult. Plating pre-treatments become essential, moreover, in order to improve the adhesive strength between the wiring circuit 44 and the metal bumps 47 formed thereon.